1. Field of the Invention
The present invention relates generally to devices for controlling light characteristics in an optical communication system, and more particularly, to devices that control the state of polarization of light in an optical communication system.
2. Description of the Related Art
Devices that control the state of polarization of light find application in fiber optical communication systems, as well as in optical test and measurement equipment. Most of the devices for controlling the polarization state of light are operated manually by turning an optical fiber or other optical element, or are operated by using the electrode-optic effect in solid state devices. These polarization control devices are typically able to convert any input state of polarization to any output state of polarization. However, mechanical polarization control devices have a slow response time on the order of 1000 ms, and have large dimensions (typically several 10 cm3), and consume more than 100 mW of power to drive the motors that provide the necessary movement of optical elements.
Another method for controlling the state of polarization of light is to use a liquid crystal-based polarization controller. Such controllers allow the transformation from any input state of polarization to a limited range of output states of polarization. By way of example, one such polarization control device is made by Meadowlark Corporation, Frederick, Colo., and uses a series of liquid crystal wave plates at fixed orientations with variable retardance to form a polarization control device. The Meadowlark device uses a modulating electric field which is perpendicular to the boundary planes of the liquid crystal device.
A problem of Meadowlark-type devices is that they are not able to convert any input state polarization to any output state of polarization. Additionally, this type of device also has a relatively large power requirement as well as a large response time.
Briefly, the present invention comprises, in one embodiment, a polarization control device, comprising: a liquid crystal device containing a liquid crystal material with an optical anisotropy with substantially homeotropically aligned molecules when in a neutral electrical field state; a plurality of electrodes for applying an electrical field to change an orientation of the molecules of the liquid crystal material from their alignment in the neutral electrical field state; a circuit for applying a voltage to the electrodes; and an optical system for directing light through the liquid crystal material.
In a further aspect of the present invention, the liquid crystal device includes two parallel alignment plates with the liquid crystal material disposed therebetween; and wherein the electrodes comprise at least one electrode disposed adjacent one of the alignment plates on one side of the directed light, and at least another of the electrodes disposed adjacent the other of the alignment plates on an opposite side of the directed light, and energized to generate an electrical field substantially parallel to the alignment plates, but with an electrical field component perpendicular to the alignment plate.
In yet a further aspect of the present invention, a polarization detector is included that is disposed to receive light transmitted through the liquid crystal material and to generate a signal dependent on the polarization of the light received from the liquid crystal material; and a feedback circuit for receiving the polarization-dependent signal, generating a control signal, and providing that control signal to the circuit for applying a voltage to the electrodes, to control a voltage characteristic of the electrodes in accordance therewith.
In a further embodiment of the present invention, a method for polarization control is provided comprising the steps of: providing a liquid crystal device that contains a liquid crystal material with homeotropically aligned molecules when in a neutral state; causing an electrical field to be generated across the liquid crystal device; and directing light through the liquid crystal material.